[Sync] Pass a TaskRunner into SBHI/SBHC, not a thread or message loop.

components/sync/driver/glue/sync_backend_host_impl.h

Index: components/sync/driver/glue/sync_backend_host_impl.h
diff --git a/components/sync/driver/glue/sync_backend_host_impl.h b/components/sync/driver/glue/sync_backend_host_impl.h
index 96849f54a8be5ca24db0778e42d65700309e779c..4035e753af05edf4163341499ad2cdc1a99eab53 100644
--- a/components/sync/driver/glue/sync_backend_host_impl.h
+++ b/components/sync/driver/glue/sync_backend_host_impl.h
@@ -71,7 +71,7 @@ class SyncBackendHostImpl : public SyncBackendHost, public InvalidationHandler {
   // SyncBackendHost implementation.
   void Initialize(
       SyncFrontend* frontend,
-      base::Thread* sync_thread,
+      scoped_refptr<base::SingleThreadTaskRunner> sync_task_runner,
       const WeakHandle<JsEventHandler>& event_handler,
       const GURL& service_url,
       const std::string& sync_user_agent,
@@ -294,8 +294,8 @@ class SyncBackendHostImpl : public SyncBackendHost, public InvalidationHandler {
 
   SyncClient* const sync_client_;
 
-  // A pointer to the sync thread.
-  base::Thread* sync_thread_;
+  // The task runner where all the sync engine operations happen.
+  scoped_refptr<base::SingleThreadTaskRunner> sync_task_runner_;
 
   // The UI thread's task runner.
   const scoped_refptr<base::SingleThreadTaskRunner> ui_thread_;